Overview
The 12th Illinois Infantry (Three Months’ Service) was an infantry regiment organized at Springfield, Illinois, for Union service at the outset of the Civil War. Mustered in for a three-month term in 1861, the regiment was primarily assigned to garrison duty at Cairo, Illinois, a strategic location at the confluence of the Ohio and Mississippi Rivers. The unit served under the Union Army in the Western Theater during its brief period of service.
This unit is distinct from the 12th Illinois Infantry (Three Years’ Service).
Organization & Service
The 12th Illinois Infantry (Three Months’ Service) was organized at Springfield, Illinois, in April 1861, with Colonel John McArthur commanding, assisted by Lieutenant Colonel Augustus L. Chetlain and Major William D. Williams. Shortly after its organization, the regiment was ordered to Cairo, Illinois, where it formed part of the garrison alongside the 8th, 9th, and 10th Illinois Infantry regiments.
During its three months of service, the regiment’s duties consisted mainly of fatigue work, including constructing barracks, clearing parade grounds, and building fortifications and a redan earthwork to defend the confluence of the Ohio and Mississippi Rivers. The regiment’s only field operation occurred when a battalion from the 12th joined a march into the swamps near Commerce, Missouri, in pursuit of Confederate forces under Jeff Thompson. The regiment was mustered out of service on August 1, 1861.
Research This Regiment Further
If you’d like to explore this unit’s history in more depth, regimental histories and Civil War reference works offer valuable detail.
Engagements & Campaigns
- Garrison duty at Cairo, Illinois (April–August 1861)
- Expedition into the swamps near Commerce, Missouri, in pursuit of Jeff Thompson
Casualties
Specific casualty figures for the 12th Illinois Infantry (Three Months’ Service) are not detailed in available primary sources. The regiment’s service was primarily garrison and fatigue duty, with limited field operations.
Field Officers & Commanders
- Colonel John McArthur
- Lieutenant Colonel Augustus L. Chetlain
- Major William D. Williams
Regimental Roster
The full roster of the 12th Illinois Infantry (Three Months’ Service), including officers and enlisted men, is available for genealogical and historical research. This roster provides names, ranks, and additional service details where recorded.
To view the complete roster, visit the 12th Illinois Infantry (Three Months’ Service) Roster page.
Sources & References
- Official Records of the Union and Confederate Armies, Series I
- Report of the Adjutant General of the State of Illinois
- Dyer, Frederick H., A Compendium of the War of the Rebellion